On‐Surface Debromination of 2,3‐Bis(dibromomethyl)‐ and 2,3‐Bis(bromomethyl)naphthalene: Dimerization or Polymerization?

Abstract We describe the on‐surface dehalogenative homocoupling of benzylic bromides, namely bis‐bromomethyl‐ and bis‐gem‐(dibromomethyl) naphthalene as a potential route to either hydrocarbon dimers or conjugated polymers on Au(111). While bis‐gem‐(dibromomethyl) naphthalene affords different dimers with naphthocyclobutadiene as the key intermediate, bis‐bromomethyl naphthalene furnishes a poly(o‐naphthylene vinylidene) as a non‐conjugated polymer which undergoes dehydrogenation toward its conjugated derivative poly(o‐naphthylene vinylene) upon mild annealing. A combination of scanning tunneling microscopy, non‐contact atomic force microscopy and density functional theory calculations provides deep insights into the prevailing mechanisms.


Reagents and Solvents for Synthesis
All reagents and solvents were obtained from commercial suppliers (SigmaAldrich Laborchemikalien GmbH, TCI Deutschland GmbH) and used without further purification. Deuterated solvents for NMR analysis were purchased from SigmaAldrich Laborchemikalien GmbH.

Column Chromatography
Flash column chromatography was carried out using silica gel (grain size 0.04-0.063 mm) produced by SigmaAldrich Laborchemikalien GmbH. As mobile phase the solvents named in the synthetic procedure were used. For thin layer chromatography Polygram Sil g/UV 254 plates from Macherey Nagel were used and examined under UV-light irradiation (254 nm and 365 nm).

Nuclear Magnetic Resonance Spectroscopy
All NMR spectra were recorded in deuterated solvents (CD2Cl2) at room temperature (if not stated otherwise) on a Bruker Avance III (300 MHz), Bruker Avance III (500 MHz) or Bruker Avance III (600 MHz). 13 C NMR spectra were measured proton decoupled if not stated otherwise. Chemical shifts δ are reported in part per million (ppm) and coupling constants J in Hz. All spectra were referenced to solvent signal. [1] For the multiplicities, the following abbreviations are used: s = singlet, d = doublet, t = triplet, m = multiplet. The spectra were processed and integrated using ACD/Spectrus processor.

TGA/DSC
Thermogravimetric analysis and differential scanning calorimetry-analysis were done using a Mettler Toledo TGA/DSC1 Star e System. The ASCII files were exported and visualized by Origin Pro 2021.

X-ray Single-Crystal Structure Analysis
X-ray single-crystal structure analyses were measured on a Bruker Smart APEX-II Quazar Area detector. Diffraction intensities were corrected for Lorentz and polarization effects. An empirical absorption correction was applied using SADABS [2] based on the Laue symmetry of the reciprocal space. Hydrogen atoms were either isotropically refined or calculated. Structure was solved with SHELXT-2018/2 (Sheldrick 2015) [3] and refined against F2 with a Full-matrix least-squares algorithm using the SHELXL-2018/3 (Sheldrick, 2018) [4] software.

On-Surface Characterization Parameters Sample Preparation
The Au(111)/Ag(111)/Cu(111) crystals that were purchased from MaTeck (Germany) and were cleaned through combined sputtering and annealing cycles. The precursors were deposited onto the single crystal surfaces by a commercial molecule beam evaporator purchased from Kentax (Germany). Reactions were triggered by annealing the sample at appropriate temperatures for 30 min. The samples were transferred into an STM/AFM scanner where the base pressure is better than 1.0 × 10 −10 mbar.

STM/AFM and STS Measurements
All of the STM/AFM images were obtained at 4.6 K with a commercial LT-STM/AFM (Scienta Omicron, Germany). The tip was grounded, and the voltage bias was applied to the sample for STM imaging and STS characterization under the constant current mode. The STS spectra (dI/dV) were acquired using a lock-in amplifier (Vrms = 20 mV). The AFM images were obtained under constant-height mode by using a tuning fork force sensor with CO-functionalized tips. The resonance frequency of the force sensor is approximately 28 kHz. Vibration amplitudes of 50-69 pm were used for all the AFM images. The quality factors are 16 k-28 k.

Computational Parameters Density Functional Theory (DFT) Calculations
All density functional theory (DFT) calculations were performed by the Vienna Ab-initio Simulation Package (VASP) together with Atomic Simulation Environment (ASE). [5] The electron-ion interactions were described by the projected augmented wave potentials and the Perdew-Becke-Ernzerhof functional (PBE) of generalized gradient approximation (GGA) was employed to treat exchangecorrelation interactions. [6] The Van der Waals interactions were considered by the DFT-D3 method developed by Grimme. [7] The cutoff energy for the plane wave was set as 400 eV. The periodic image interactions were avoided by employing a 20 Å vacuum layer. The search for the transition states was conducted by a combination of Climb-Image Nudged Elastic Band (CI-NEB) method and Dimer method. [8] First, 10 images were inserted in between the initial and final state by CI-NEB method. Second, the central image was further adopted as the input for the Dimer calculation in order to achieve high precision. The structure of local minima and the saddle points were optimized until the average atomic force was lower than 0.02 eV/Å. The coinage metal surfaces were modelled with periodic slabs with four atomic layers, in which atoms in bottom two layers were constrained. The Brillouin zone was modelled by gamma centre Monkhorst-pack scheme, [9] in which a 2 × 2 × 1 gird was used for all calculations.

AFM Images Simulations
The AFM simulation images are obtained on website: http://ppr.fzu.cz/. The Probe Particle Model is used in simulation of Highresolution atomic force microscopy (AFM), scanning probe microscopy (STM) and inelastic tunneling microscopy (IETS) images using classical forcefileds. [10] Files of three repeated units of polymer 12 and 4 were uploaded to obtain the AFM simulations. For both cases, the smallest tip heights were chosen for better resolution.